1. Field of the Invention
This invention relates in general to shaft couplings, and in particular to connections between pipe sections in a drill string.
2. Description of the Prior Art
Oil and gas wells are drilled with a rock bit, mounted on the bottom of a drill string. A drill string, which may be several miles long, is a series of pipe sections, each about thirty feet long. The upper end of the drill string is rotated by a rotary table, and, as the drill string rotates, the rock bit is rotated to cut into the earth.
During drilling, the torque required to rotate the drill string may suddenly increase for some reason, such as the sides of the hole collapsing. If the torque on the drill string exceeds the makeup torque of the tool joints, downhole makeup may occur, producing additional stress which may exceed the yield strength of the tool joints between pipe sections. The tool joints may be damaged, and may even separate. The drill string must then be removed from the hole, which is a costly, time-consuming, and dangerous operation. If the lower part of the drill string is lost due to separation, an expensive fishing operation must be performed, or the hole may even have to be abandoned.
In order to prevent such damage, safety joints are sometimes inserted into the drill string at various intervals. A safety joint performs as a normal tool joint at normal torque, but when the torque becomes excessive shear pins in the safety joint are sheared and the safety joint becomes a swivel coupling. Such safety joints are described in U.S. Pat. No. 4,064,953 (Collins) and in U.S. Pat. No. 1,414,207 (Reed)
After the shear pins in a safety joint have been sheared by excessive torque, the drill string must be removed from the hole, and the shear pins replaced, before normal rotary drilling can resume. Such an operation is expensive, time-consuming, and dangerous to personnel on the drilling platform.